Caterloopillar is the name given to a family of spaceships which provided a general solution and a script to generate all supported special cases allowed by current hardware/software state of the art (i.e. golly at the time) and lower than c/4 (although the script is not working for some special subset which requires more recipes i.e. there is a subspace which is still not solved technically but the algorithm to generate the recipes is clearly given and shown to be working statistically and the combination of the recipes into a spaceship was provided for many cases and open publicly). The design is containing two major parts - the front and the back. In Caterloopillar each part has the properties of a universal constructor, and thus each part is constructing the building blocks of the other part, while also reading and moving a construction tape. As the name suggests, this creates somewhat paradoxical design, similar to the "strange loop" concept.

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History

The general idea of adjustable speed spaceship was proposed by David Bell in October 2006 on the LifeCA mailing list[1], and possibly earlier and independently by Jason Summers. Hartmut Holzwart revived discussion about the idea in September 2014 on the ConwayLife.com forums.

During those discussions, Alexey Nigin in 2014 proposed a a novel way of construction that would avoid helices, instead it would use two streams upward and downward, that create each other using slow salvos.[2] Surprisingly enough the final design[3] of the first Caterloopillar was very close to his description.

Alexey Nigin's initial design proposal was purely theoretical. The first example showing that the idea could be efficiently implemented was found by Michael Simkin a few days after the initial suggestion[4].

The name was also extensively discussed, with suggestions including strange loop, strange-caterloopillar, pushmi-pullyou and caterloop, finally arriving at Caterloopillar.

Implementation

The first Caterloopillar was constructed by Michael Simkin and completed on April 9, 2016; this Caterloopillar has a speed of c/8. Caterloopillars with many other speeds can be constructed using a slightly modified design. The major limitation of this implementation is upper bound of speed c/4, as the signal transferring is done by converting a still life into a glider and back, thus limiting the moving speed of the tape to c/4 (the speed of the glider).

Simkin went on to construct Caterloopillars of other various speeds[5], the first three being c/9, c/11 and c/12 orthogonal, respectively. One of the most notable caterloopillars is a 31c/240 one smaller than the centipede, the previous record holder which was not based on caterloopillar technology.